Every 10 litres of biodiesel leaves about 1-1.5 litres of crude glycerin, which holds 0.91 kg of real glycerol but also unused lye, soap and a large share of the excess methanol, so it is caustic and poisonous before it is useful. Store it sealed and labelled, and use it only after the methanol is dealt with; do not compost it, burn it as fire-starter or pour it into water.
Part of Biodiesel Production
Crude glycerin is the dense lower layer drained from a biodiesel batch: glycerol mixed with lye, soap, methanol and water.
How Much And What Is In It
The reaction frees one glycerol for every three esters. From the mass balance in Triglyceride + Methanol → FAME + Glycerin: 95.7 g of glycerol per litre of oil, 10.4% of the oil’s mass. For 10 litres of biodiesel (8.8 kg) that is 0.91 kg of pure glycerol, 0.72 litres. The parent’s 1-1.5 litres of crude layer is bigger than that because the layer also carries:
- Unused lye (the catalyst, at least some of it);
- Soap, made from free fatty acids and from any water in the batch;
- Methanol, much of the excess (the batch used about 74 mL more than the 126 mL needed per litre of oil), which is both poisonous and flammable;
- Water.
NaOH-made glycerin tends to set solid when cool; KOH-made stays liquid (parent).
The Parent’s Uses, Corrected
The parent lists four uses. They need qualifying because of what the layer contains:
| Parent’s use | Problem with crude glycerin | Safer way |
|---|---|---|
| Soap making: “add water and heat gently” | heating releases methanol vapour; the alkali content is unknown | Soap Making from Glycerin |
| Composting as a carbon source | caustic and toxic to the compost’s microbes; methanol is a poison | only after methanol is removed and the alkali neutralised |
| Fire starter | the crude layer holds water, lye and methanol; it burns poorly and the methanol flashes | not recommended |
| Do not dump in waterways | correct | keep contained |
Neutralising the lye with vinegar is possible: for a 20-litre batch that used at most 110 g of NaOH (2.75 mol), at most 2.75 × 60.05 = 165 g of acetic acid is needed, which is 3.3 litres of 5% vinegar (Vinegar Production gives vinegar at 5-10%). That is an upper bound, because part of the lye was used up. Neutralising does not remove methanol.
What You Need
- A sealed, labelled, alkali-resistant container (Container Selection)
- Gloves and goggles
- A place outdoors, away from drains, wells and flame
Method: Store It Safely
Step 1: Drain it from the settling vessel into the marked container (Settling & Separation).
Step 2: Close the lid and store it cool and out of reach of children, labelled “CAUSTIC – POISON – CONTAINS METHANOL”.
Step 3: Decide on a use only when you can do it safely: recovering the methanol, soap, or contained disposal.
Common Mistakes
- Treating it as harmless because it is “just glycerin.”
- Pouring it onto soil or into a drain. Methanol and lye reach water; glycerin itself also depletes oxygen in streams (parent).
- Breathing the vapour when warming it.
Caustic, toxic and flammable
The layer can burn skin, and methanol in it is absorbed through skin and vapour and blinds (Methanol Hazards). Wear gloves and goggles, handle outdoors, and wash splashes at once with a great deal of water (Spill Response).
What’s Next
- Learn next: Soap Making from Glycerin
- Combine with: Other Uses